It is well known that a load cell comprising a strain gauge formed by the use of a vapor deposition technique or a sputtering technique is manufactured by applying a heat-resistant high molecular material such as, for example, polyimide, on a surface of a strain inducing element to form an electrically insulating film, subsequently forming a thin film of nickel-chromium alloy or tantalum nitrate on a surface of the electrically insulating film by the use of a vapor deposition technique or a sputtering technique and finally photo-etching the thin film to form a resistance pattern.
The load cell particularly used in a weighing apparatus such as a scale comprises four strain gauges which are electrically connected to form a bridge circuit. Therefore, where all of the strain inducing elements vary uniformly with a change in temperature, temperature dependent changes in outputs from the respective strain gauges can be counter-balanced with each other. However, where an article of a temperature considerably different from a room temperature such as, for example, a frozen food material, is placed on a load supporting plate connected with one end of the load cell, a temperature gradient occurs in strain inducing elements to such an extent as to result in varying resistances of the respective strain gauges according to a temperature dependent coefficient of resistance. Once this occurs, the variations in resistance among those strain gauges can no longer be counterbalanced with each other by the bridge circuit.
By way of example, in the case of the load cell comprising strain gauges prepared by forming an electrically insulating film of polyimide on a surface of a strain inducing element and then sputtering a thin film of tantalum nitrate on a surface of the electrically insulating film, one of the resultant strain gauges may have a temperature dependent coefficient of resistance which is about -40 PPM/deg(.degree.C.) and, consequently, when this load cell is applied in a weighing apparatus, a problem would occur in that an measurement error in the order of about 200% per degree at maximum tends to occur.
The present invention has been devised with the foregoing problem taken into consideration and has for its object to provide a load cell wherein strain gauges each having an extremely small temperature dependent coefficient of resistance are integrally formed.